Rotors in industrial electrical power generators undergo high rotational speeds creating large centrifugal forces that stress the rotor. Any imbalances in the rotor will cause vibration. Similarly, the electrical currents in the rotor generate thermal stresses in the rotor field windings. The uneven distribution of these thermal stresses can create mechanical moments that bend and unbalance the rotor. Current created thermal stresses in the rotor can cause the rotor to vibrate.
Rotor field windings carry large currents. Rotors and their windings must withstand substantial thermal strain energy during generator operation. At normal operating speeds, the rotor typically rotates at speeds of 3,600 revolutions per minute (RPM) and faster. In addition, the rotor may have several operating speeds such as 1500, 1800, 3000 and 3600 RPM. Thus, the rotor must be dynamically balanced at several high rotational speeds. Any imbalance in the rotor may destabilize the rotor and cause unwanted vibration. Vibration significantly contributes to the fatigue and wear of the rotor. Moreover, the rotor cannot be safely operated if the vibrations exceed certain relatively low threshold limits.
Thermal vibrations in a generator rotor occur because the large currents carried by the field windings heat and deform the windings and rotor. The currents electrically excite and heat the windings. As they are heated, the windings expand. As the windings expand adjacent turn layers of the winding bind together, and bind against the sides of the rotor slots, the underside of the wedges in the rotor slots, the underside and lip of the retaining rings, and against the end of the end blocks. This thermal expansion of the windings generates slot friction forces principally on the top turn of the winding. Similarly, restraining forces build up as the top turn of the winding stack rubs against steps and discontinuities in the slot and on the retaining ring. The top winding turn experiences most of the thermal stresses generated by the rotor currents. In prior art field windings, these thermal stresses on the top turn propagate through the entire winding stack due to friction between the turns of the winding.
The stresses in the rotor field winding generated by thermal expansion can be categorized into five primary stress mechanisms within the rotor. These stress mechanisms are: (1) the end winding block which abuts against the expanding winding, (2) the coefficient of friction between the retaining ring and the top winding turn, (3) steps and other discontinuities in the inner surface of the retaining rings that bind the top winding turn, (4) the coefficient of friction between the rotor slot and the winding stack, and (5) steps and other discontinuities on the underside of the rotor slot wedges and insulation layer that deform, bind and grind the top winding turn. These stress mechanisms are not entirely predictable and generate stresses in a somewhat random manner. The stresses can vary across the length of the rotor and can vary downward between individual windings in the stack. Moreover, the stresses can also vary between the top turn of the winding and the underlying winding stack.
Prior techniques for overcoming thermal stresses and reducing thermal vibrations in rotors include cooling the rotor, imparting a high flexural stiffness to the rotor, employing body mounted retaining rings that do not engage the rotor spindle, orienting all insulating components of the rotor in 180 degree opposing positions, and coating the upper surface of the top turn. A stiff rotor resists bending and deformation and, thus, dampens thermal vibration in the rotor. Similarly, a body mounted retaining ring effectively stiffens the rotor to reduce the thermal stress sensitivity of the rotor. A body mounted retaining ring also does not transmit bending stresses to the small diameter spindle of the rotor. In addition, orienting the insulating components of the rotor to directly oppose one another theoretically cancels out thermal stresses in the rotor to reduce the bending moments on the rotor. Finally, coating the upper surface of the top winding theoretically allowed the winding to slide with respect to the insulation layers, wedges and retaining rings.
These prior art techniques have their faults. The cooling and high stiffness techniques are practical provided that the rotor always operates below threshold temperatures and stresses. All too frequently, rotors are operated beyond these threshold limits and excessive thermal vibration permanently damages the rotor. Similarly, body-mounted retaining rings cannot be economically retrofitted on older rotors having the traditional retaining rings that mount on the spindle and rotor body. In addition, body-mounted retaining rings add significantly to the cost and complexity of the rotor design.
Opposing insulating components are extremely difficult to design and nearly impossible to maintain over the life of the rotor. Every rotor has unpredictable and random steps and discontinuities at, for example, butt joints and overlaps of insulating materials. Accordingly, it is impossible to design opposing insulating components that have equal but opposite stresses. Similarly, fatigue and creep mechanisms over extended periods of time randomly alter the thermal strain energy and force distribution in the field winding. During its lifetime, the rotor will react to thermal energy and vibration in a random and unpredictable manner.
Similarly, coating the upper surface of the top winding is ineffective because this soft copper surface deforms underneath the insulation layer, slot wedges and retaining rings. The deformities in the upper surface of the top winding bind against the insulation layer. Accordingly, these deformities prevent the top winding from sliding with respect to the insulation layers regardless of any coating between the top winding and the insulation layer.